1. Field of the Invention
The present invention relates to the removal of concentrations of heavy metals from aqueous systems, such as from electroplating baths, mine water discharge, seeps from chemical work drops, and in particular aqueous soil systems (which represent a subset of aqueous systems) where the soil is derived from urban soils, soils in vicinity of areas of heavy automobile usage, soil with agricultural pesticide residues or runoffs containing pesticide residues, as well as soil system's characteristic of those found at present or former chemical manufacturing facilities, mines, Department of Defense or Department of Energy sites, and facilities receiving demolition debris, hazardous wastes, or mixed wastes. Such soil systems are normally considered as aqueous soil systems since they contain some heavy metals, some mount of water, and large amounts of particulate material, or can be easily made into aqueous soil systems with the addition of water, whereas other types of aqueous systems contain heavy metals, solubles and limited amounts of particulate material.
Heavy metal contaminants include chromium, lead, mercury, strontium and nickel cadmium. These metals have become contaminants in aqueous systems as the results of activities including chemical manufacture, smelting, electroplating, wood treating, and covered metalworking operations, to name only a few areas, where such metals are used. Heavy metals also include cobalt, thorium, uranium, and the transuranics, particularly where created as a by-product of uranium, thorium and plutonium beneficiating ore milling and smelting to create commercial or military reactor target cores (fuel rods), industrial and medical radioisotopes, depleted uranium, counterweights, shielding, and the like. When found in soils these heavy metals can be removed by soaking the contaminated soil with water and utilizing modern remediation processes.
Commercial remediators presently use in situ vitrification, acid soil washing, chemical chelating techniques and stabilization/solidification to treat or fix heavy metal contaminants. These techniques generally utilize large volumes of water, have a significant adverse impact on soil and flora and are increasingly expensive. A large number of commonly used remediation technologies, including burning and incineration, and vapor extraction/soil venting, are not by their nature feasible for heavy metal removal. A heavy metal removal technology with the potential to use less water and be less destructive to soil structures, if soil is present, and natural flora has long been sought.
This invention relates to improvements relating to heavy metal removal from aqueous systems for waste volume reduction and removal of contaminants including lead, mercury, cadmium, uranium, cesium, thorium, cobalt, and other similar heavy metals.
The benefication of aqueous soil systems containing radioactive or non-radioactive heavy metals and the removal of these contaminants can also be achieved using the process of this invention. This inventive process can be combined with a variety of other methods, including the use of one or more standard, conventional techniques such as evaporation, precipitation, clarification, thickening, sludge filter pressing sand-bed filtration, fixed and moveable bed ion exchange, activated carbon purification, and soil neutralization techniques. To manage effluents which may be generated, additional conventional steps such as sludge solidification and organic oxidation may also be utilized so as to control fugitive emissions and minimize uncontrolled losses of other potential pollutants.
The organoclay-based process of this invention can be utilized in conjunction with most of the above techniques. The treatment process is targeted to treat relatively low levels of heavy metal contamination. Many prior art processes based on expensive and complicated operations, can be generally suited for fixed and centralized plant installations, but are not generally satisfactory for certain field soil remediation applications, where climactic conditions can be adverse. A simple system to remove heavy metals with relatively problem-free transportability and ease of operation has therefore long been desired.
2. Description of the Prior Art
The development of cost-efficient routes to remove concentrations of heavy metals from aqueous systems is of commercial interest to a large variety of North American private, commercial and public remediators. By aqueous system it is meant a system containing water and heavy metals, where the heavy metals are in ionic or complexed form, or both. Complexed form means the heavy metal ion has been electrically neutralized by one or more counter ions. Other constituents may also be present, such as soils, solubles, industrial wastes and fillers. Soil can be made up of all sorts of rocks, sands, minerals, decomposing organic mater, fossil remains and similar components.
There are many sites where heavy metal pollution requiring abatement has been discovered. Not only are there over 1,200 targeted NPL (National Priority Listing) "Superfund" sites, but there are over 50,000 further identified "State Superfund", DOD, and other such sites awaiting treatment. Recent estimates suggest the cost to remediate all of these sites will reach well into multiples of tens of billions of dollars.
As the nation continues to struggle with the cost of funding this cleanup, a consensus has emerged among lawmakers, the Executive Branch and concerned state government agencies to focus on implementing cleanups with the lowest possible cost, using practical, inexpensive and available technology. Last year, preliminary agreement was reached among government agencies and an insurance consortium to develop a pooled insurance fund and simplified review procedures to support remediation activities at contested sites. It is likely that this plan or something similar will be enacted during the present Congress, and that such an agreement will support the focus on lost efficiency and practicality with an improvement in the regulatory climate. It is estimated that five billion dollars will be spent on the site remediation during 1995 alone. It is hoped that such a focus will reverse the present trend toward costly and unproductive legal actions challenging the imposition of liability and high legal costs.
Outside the "Superfund-type" pollution area, application sites for heavy metal removal technology in industrial soil site remediation, mine remediation and DOD mixed waste sites include the following:
(a) Approximately one hundred thousand abandoned and active mine sites in the Western states await cleanup, at a cost not yet estimated. A portion of funds to be spent for remediation must be allocated to process heavy metals found in mine pits and ponds, soils, and mine waste (including overburden). A number of technology companies have commenced operations around these old, sometimes historical, mining centers, and will now attempt to "mine" the flow of federal, state, private and Superfund monies directed towards site cleanup. PA1 (b) DOD "Mixed Wastes", which are most often comprised of radionuclide salts, organics and heavy metal industrial wastes. Large volumes of radionuclide- contaminated water and mixed stored at DOD sites await treatment. For example the Hanford, Wash. site has been the subject of feasibility projects which have themselves cost $3 billion dollars, and have resulted in the development of a cost estimate for actual clean-up which could exceed $35 billion dollars for this one location alone. PA1 (a) contacting an aqueous system containing heavy metals dissolved in ionic and/or complexed form with one or more organically modified smectite-type clays for a period of time sufficient for the heavy metals to be sorbed by the organically modified smectite-type clay; and PA1 (b) removing the organoclay/heavy metals sorbed complex from the aqueous system.
The separation, concentration, immobilization and disposal of heavy metal contaminants on a cost-efficient basis continues to be studied by both government agencies and private industry. Those involved in the disposal industry have also been searching for a variety of waste clean-up agents.
U.S. Pat. No. 4,167,481 describes that a cation exchanging clay, such as bentonite or montmorillonite, which is combined with a polyamine, such as tetraethylenepentamine, and the combination used in contact with solution to remove several metal ions, such as mercury, cadmium, copper, zinc, nickel and cobalt. The patent mentions that the addition of polyamines, which are capable of forming stable cationic complexes with most metal ions, to metal-containing aqueous solutions, has a pronounced synergistic effect on the adsorption of these metals. In particular, the patent teaches that tetraethylenepentamine in combination with phyllosilicates such as bentonite and montmorillonite is effective in reducing the concentration of copper and mercury to very low values, sometimes down to 10 ppb (parts per billion) or less. The patent further discloses formation of a cationic polyamide complex which is easily adsorbed in the cation exchanger and which is stabilized by a factor of about one thousand, as compared to the stability of the complex in solution. Recent U.S. Pat. No. 5,114,892 uses a mixture of bentonite clay and sodium salts of polyphosphoric acids to confine seawater containing double-charged cations of zinc, cadmium and nickel.
Both unmodified smectite clay and organically modified smectite clay have been used to remove organic contaminants from fossil ash and streams--see U.S. Pat. Nos. 4,740,488 and 4,726,710. Bentonite has been used to neutralize double-charged cations naturally present in seawater and individual waste--see U.S. Pat. No. 5,114,892.
U.S. Pat. No. 4,386,010 discloses that porous calcined mineral substrates treated with organic-substituted quaternary ammonium or phosphonium compounds are effective sorbents for the purification of a variety of aqueous liquids. The patent describes a process for removing organic and inorganic contaminants from liquids which comprises treating the solution with particles of a porous calcined mineral and quaternary ammonium or phosphonium compound. Exemplary minerals described are high-calcium bentonites, acid-activated bentonites and calcined heat-treated grades of attapulgite clay, with the temperatures of the heat treatment used generally above about 150.degree. C. U.S. Pat. No. 4,444,665 teaches the use of attapulgite clay rendered substantially non-swelling, high-calcium bentonites, and acid-activated bentonites treated with a large-molecule quaternary ammonium compounds to remove organic and inorganic contaminants from process liquors.
Recent European Patent, EP 0,560,423 A1, discusses the use of an organoclay as an agent for fixing organic and inorganic impurities along with a clay platelet crystalline growth agent. The method, according to the invention, is characterized by the organic and inorganic impurity-containing material to be fixed to and/or enclosed in a clay material to be formed in situ. One example of the patent discusses a radioactive cation, Cs.sup.137 and in particular, teaches its fixation with an organoclay and hardening agent.
U.S. Pat. No. 5,028,338 discloses a process for immobilizing environmental noxious organic, particularly aromatic, substances by absorbing them on a layered clay material modified by exchange with a quaternary ammonium compound, reducing the spacing of the layers of clay, and the mixing the clay material with a hardenable inorganic binder.